Septic tank or treatment system for a new house in Ireland

Arrange a site assessment before choosing a septic tank or treatment plant. The ground conditions, available space and proposed house determine which system your designer can specify.

Your assessor and designer must use the applicable Environmental Protection Agency (EPA) Code of Practice and check the whole layout. A treatment plant cannot make every unsuitable site buildable.

How the two options differ

A septic tank system has two parts: a tank and a percolation area. Solids settle in the tank. The liquid drains through underground pipes into the soil, which treats it before it reaches groundwater.

Where the assessment allows, a filter or mechanical plant provides further treatment. The design must also provide a suitable area for treated water to enter the ground. A constructed wetland is a purpose-built planted bed that treats wastewater. A media filter contains material on which microorganisms grow to treat it; a willow bed is a specially designed treatment bed planted with willows. These and soil or sand filters are treatment-system options, not ordinary garden features. The EPA Code of Practice groups the options as:

• Septic tank plus percolation area, where the tank provides initial treatment and the soil provides further treatment.
• Secondary treatment system plus soil polishing filter. A packaged plant or filter treats the water first, and the soil finishes the job.
• Tertiary treatment system plus infiltration area. An extra stage of treatment before the water reaches the soil.
• Drip dispersal, which spreads treated water through small pipes. The Code allows it on the slowest-draining soils.

Have the site assessed first

You can’t pick a system and then find a site for it. The site suitability assessment comes first, and the result narrows your choices. A planning application for a new house must show the site suits the proposed system. The design must meet:

• the Code of Practice,
• the Building Regulations (Technical Guidance Document H), and
• Standard Recommendation S.R. 66 with the EN 12566 series.

S.R. 66 guides selection and minimum performance of wastewater treatment products tested under EN 12566, the European standards series for small wastewater systems. The National Standards Authority of Ireland published the amended S.R. 66:2015+A1:2026 in September 2026. Ask your designer to check the current product and design requirements.

The 2021 Code applies to site assessments and installations on or after 7 June 2021. The 2009 code can still be used for work started before that date, or where permission was applied for before it. For a new application today, assume the 2021 code.

Not every site is suitable, and the planning authority makes the final decision. A suitably qualified site assessor tests your site; ask your designer or the council for one. See planning permission for a one-off house.

What the assessor measures

The assessor reviews maps and existing site information (a desk study), digs a trial hole and runs percolation tests. The Code’s flow chart asks, among other things, whether the ground rises or falls no more than 1 metre over 8 metres horizontally (a slope of 1 in 8), whether the separation distances are met and whether the minimum depths of soil above bedrock and the water table are met. The EPA publishes an electronic Site Characterisation Form that records the results.

The key number is the percolation value (PV), in minutes for water to drop 25 mm in the test hole. A low PV means fast-draining sand or gravel. A high PV means slow-draining clay. Table 6.4 of the Code says which systems each range allows:

System	Allowed value
Septic tank and percolation area	3 to 50
Secondary treatment and soil polishing filter (gravity flow, trenches 500 mm wide)	3 to 75; if installed at ground surface, subsurface test must also be 3 to 90
Secondary treatment, low-pressure pipe, trenches 300 mm wide	3 to 90
Secondary treatment with drip dispersal	3 to 120
Tertiary treatment and infiltration area	3 to 75; if installed at ground surface, subsurface test must also be 3 to 90

The surface test measures drainage near ground level; the subsurface test measures it deeper down. For the rows with two ranges, installing the infiltration area at ground surface needs a surface result of 3 to 75 and a subsurface result of 3 to 90. Ask the assessor which test results and installation level your design uses. Gravity flow means water moves without a pump; low-pressure pipes use a pump to distribute the treated water.

Below 3, the water moves too fast to be treated. You may be able to import slower-draining soil, or discharge treated water to a watercourse such as a stream, which needs a Water Pollution Act licence from the council. Above 120, the site is unsuitable for discharge to ground.

A percolation value of 40 falls within the septic-tank range; 60 rules out that option but falls within some treatment-system ranges. At 100, Table 6.4 leaves the drip-dispersal option, provided the rest of the assessment and design requirements pass. Low-pressure pipe trenches stop at 90. These are checks on one measurement: a passing value alone does not establish that a system is suitable.

Check soil depth as well as drainage

Depth matters as much as drainage speed. Unsaturated soil has spaces not filled with water; the required layer lies above the water table. Table 6.3 sets minimum depths of unsaturated soil or subsoil below the point of infiltration. This is usually the base of the distribution gravel, or the tubing for drip dispersal.

Your assessor identifies the groundwater protection response using site evidence and the EPA and Geological Survey vulnerability mapping. The response is a code such as R1 or R2¹, and a higher number means greater risk to groundwater. It determines the depth needed, so ask your assessor which code your site has:

• Septic tank percolation trenches: 1.2 m for responses R1 or R2¹; 2.0 m for R2², R2³, R2⁴ and R3¹. They are not acceptable for R3².
• Polishing filters after secondary treatment: 0.9 m, 1.2 m or 1.8 m, depending on the response.
• Drip dispersal at a percolation value above 75, and qualifying tertiary systems: 0.6 m, 0.9 m or 1.2 m, depending on the response. The tertiary system must meet the specific bacterial-treatment performance in Table 6.3, not just be called tertiary. Ask the assessor whether your design qualifies and what additional conditions apply.

Shallow bedrock can therefore rule out a septic tank while leaving a treatment system for the assessor to consider. You do not need to choose a response category yourself; ask the assessor to explain which applies and how the design meets it.

Leave room around the whole system

The tank and the area where treated water enters the soil both need space. The plan below shows selected minimum gaps. Up-gradient means groundwater flows from the well towards the system; down-gradient means it flows from the system towards the well. Your assessor establishes this direction, which may differ from the surface slope. Ask for a scaled layout checking all the distances in the site assessment guide, including neighbouring wells and houses.

Size the system for your bedrooms

The design population, also called population equivalent (PE), is the number of people the system must be sized to serve. The Code sizes everything from the number of bedrooms, not from the people who actually live there (Table 3.2):

Bedrooms	People (PE)
1 to 2	4
3	5
4	6
5	7

The Code assumes 150 litres per person per day. A septic tank must have a usable capacity of at least C = (150 × PE) + 2,000 litres, with a minimum of 2,600 litres. For a four-bedroom house (PE 6) that is (150 × 6) + 2,000 = 2,900 litres. The tank’s declared usable capacity must be no less than that.

The percolation trench length also grows with the number of people. Table 7.2 gives 72 m for 4 people, 90 m for 5 and 108 m for 6. So a four-bedroom house needs a longer trench than a three-bedroom house with the same soil, and the land needed grows with it. Add a bedroom later and the system may be undersized, so design for the house you might extend to.

Rainwater and run-off from paved areas must not go into the system. Grey water from washing machines, baths and showers must.

Choosing between them

Ask your assessor and designer these questions:

1. Which systems does the assessment allow on this site?
2. How much space does each option need, what ground heights or slopes does it require, and does it need electricity?
3. What will it cost to install, run and service over ten years?
4. Does the layout leave room for the percolation area away from wells, boundaries and the house?

Put the answers into your budget. Site works can change the cost a good deal when a filter or plant is needed. Get written quotes for the suitable options before you commit.

Compare the running costs as well as the installation price:

Option	Running needs
Septic tank	A gravity layout may need no power; pumped layouts need electricity. Have the tank emptied (de-sludged) at the interval suited to the number of people in the house. A basic tank has no alarm
Packaged plant	Electricity for equipment that adds air to the wastewater, or for a pump, periodic alarm checks, and servicing every 12 months or as instructed by the manufacturer. Use a trained, qualified provider. Arrange annual de-sludging or follow the manufacturer’s recommendation

A septic tank needs a percolation value of 3 to 50 and deeper unsaturated soil. Treatment layouts can allow values up to 75 to 120, depending on the design, and may need less soil depth. The assessment must establish which layout, if any, meets all the requirements.

Register the system and keep the certificate

Homeowners are responsible for registering their system. Councils handle applications through Protect Our Water. The registration fee is €50. You no longer need to re-register every five years, and existing certificates stay valid. Register as soon as possible after installation. Failure to register is an offence with a maximum individual fine of €5,000. When ownership changes, the new owner must update the register; the certificate is required when selling or transferring the property.

City and county councils inspect systems, follow up complaints and identify tanks that may be polluting. The EPA sets the National Inspection Plan. Keep your registration certificate and a documented record of de-sludging, inspections and maintenance available.

Plan emptying and servicing

You are responsible for operation and maintenance. Ask the installer for the maintenance schedule and keep it with the registration certificate.

• De-sludging. The Code’s Table 12.2 gives recommended intervals by tank size and occupancy. A 2,500-litre tank with 4 people should be emptied every 2 years. With 2 people, it is every 5 years. A 4,000-litre tank with 5 people is every 3 years. If you do not know the tank volume, the Code says to assume 2,500 litres. If your system has a separate chamber containing a wastewater pump, have that emptied too. Ask your installer whether yours has one.
• Who may do it. The 2012 regulations require a contractor with a waste collection permit. Farmers have a narrow exception for spreading on their own land. The National Waste Collection Permit Office has a search for authorised septic tank operators.
• Records. Keep the receipt for each de-sludging for five years. The Code also says to record all maintenance.
• Packaged plants. Arrange de-sludging at least once a year, or as the manufacturer instructs. Tertiary systems are serviced every 12 months, drip dispersal every 6 months, or as the manufacturer says. Keep your own checks to the alarm, a visual check of the unit, and noting odours or noises. Don’t open mechanical or electrical parts yourself.
• Wells. If you use a private well, the EPA advises testing it at least once a year, especially after heavy rain. If you suspect the system is affecting your well, have both checked by a qualified person.

Mistakes to avoid

• Ordering a tank before the assessment. The test results decide what you can install.
• Sizing for today’s bedrooms. The Code sizes by bedroom count, so a later extension can leave the system too small.
• Building the house first and fitting the system around it. The Code’s minimum distances from the house to the treatment area (7 m to the tank/plant and 10 m to the infiltration area; constructed wetlands have separate rules) limit where the house can go. See choosing a site.
• Skipping the records. You must keep the de-sludging receipts for five years.

The well and the treatment system have to be positioned together: see water supply for a one-off house.